Change 1-2 gears to lower ration gears 1967 Dart Automatic (904?)

-
Dana67Dart

Dana67Dart

The parts you don't add don't cause you no trouble
FABO Gold Member
Joined
Jul 16, 2017
Messages
15,014
Reaction score
12,359
Location
Northern Colorado
Has anyone swapped out the lower gear ratio 1-2 gears in a 904 trans? Hhow far apart do you have to take the transmission? Any special things to look for?

Thanks
 
The low gear set came in later transmissions like a 998/999, and you will need to get deep into the trans to do it.
 
you will need to remove the front pump...the two clutch pacs . Then you will see what is called the drivetrain portion of the trans...that is where you change out the gear ratio....not hard...just make sure you have all the pieces that go tomorrow with the lower gear set.
 
65Vsignet said:
The low gear set came in later transmissions like a 998/999, and you will need to get deep into the trans to do it.
Click to expand...

I was told there are gears that are a direct swap, am I misinformed?

Anyone know if there is a kit or a list of the correct parts?

70aarcuda said:
you will need to remove the front pump...the two clutch pacs . Then you will see what is called the drivetrain portion of the trans...that is where you change out the gear ratio....not hard...just make sure you have all the pieces that go tomorrow with the lower gear set.
Click to expand...

Sounds like it might be a job for a trans shop. Although I would love to rebuild a trans someday.
I'm adventurous just don't want to brick it.
 
Ok, gotcha
But then, I gotta say this; That low gear set is also a wide-ratio set, and it plays very nice with low-rpm,hi-torque engines, and with really tall hi-way gears. But
.....it doesn't play all that nice with Hi-RPM engines. But I'm guessing you already know that.
 
Last edited:
How about low compression 273? If it can have a bit more get off the line id be happy espically with a highway ratio rear.
 
Dana67Dart said:
How about low compression 273? If it can have a bit more get off the line id be happy espically with a highway ratio rear.
Click to expand...

IMO,
You'd be miles ahead if you just bring up the compression.A few more bucks,I know, but more torque, more horsepower, and more mpgs.
But , a fair bit cheaper is a slightly higher stall TC. I'm a really big fan of the 2800 for a 318, and so I'm sure it would be just as great behind the 273.
But yes, the 273 will like the lower first gear...... but sooner or later she's gotta get into second. :(

When you put that 2800 in there, the rpm will rise some 1000 rpm higher on the start-line, than it does now. And I'm guessing your torque delivery will potentially be nearly double. That's a 100% increase.
The low gear in that A998 is 2.74 ratio, compared to the A904 at 2.45. That is an 11.8% increase
Lessee; 100% versus 11.8%
Depending on the importance of hi-way fuel mileage, you could get away with less TC, But I wouldn't go less than maybe 2500, which should be some 750 rpm higher than a stock 1750 .The difference in horsepower delivery between 1750 and 2500 is; I'M GUESSING, 80 ftlbs which calculates to 50/55hp. That will boost your take-off................................big :) time...................And you get to keep the slightly closer-ratio A904
 
Last edited:
The /6 904 has a 2.76 first gear that can be transferred to a SBM 904 case. Just be sure to run the 4 pinion planetary in the front bell with a 28 tooth front & 38 tooth rear sun gear.
 
AJ/FormS

I wish I knew 1/2 of what you do. Thanks for the info. A TC would be much easier to do than the gears. The reason I am inquiring about gears is I'm in the process of building an 8 3/4 and could stay with the ~2.94 highway gears or step up one notch to ~3.23. So before I commit on a rear gear ratio (stock tire / wheel size) I figured I would investigate the other options. I have no plans to build out the engine, just want to be able to spin the tires without turning a corner on occasion. ;^) cheap thrills from a 51 year old daily driver.
 
A good set of 3.23, or even 3.55 gears with a posi would probably fit the bill for you. My suggestion would be to find a 742, or 489 case since they have bigger pinions than the 741 case.
 
as a matter of fact I'm looking for a 742 case, preferably with a date code of mid to late 66
 
oops,double post, sorry can't get rid of image

land_dyno.jpg
 
Last edited:
I am getting bits and pieces here of what you have and what you don't like about it and where you want to go.
 
Dana67Dart said:
AJ/FormS

I wish I knew 1/2 of what you do. Thanks for the info. A TC would be much easier to do than the gears. The reason I am inquiring about gears is I'm in the process of building an 8 3/4 and could stay with the ~2.94 highway gears or step up one notch to ~3.23. So before I commit on a rear gear ratio (stock tire / wheel size) I figured I would investigate the other options. I have no plans to build out the engine, just want to be able to spin the tires without turning a corner on occasion. ;^) cheap thrills from a 51 year old daily driver.
Click to expand...
land_dyno.jpg

have a look at this;
ignore the blue line.Ignore the right side numbers. Half all the other numbers.
This is now a visual representation of your 273 torque curve, up to 3000rpm.
Go to 3400(now 1700) and find the "torque" there on the red line, to be about 125 ftlbs@1700.
Go to 5600(now 2800) and find the torque there to be about 205 ftlbs@2800.
Now some math; your current first gear TM is 2.45x2.94=7.20.
And you are currently taking off at 7.2x 125ftlbs=900# road torque.
With a 2800TC, it would be 7.20 x 205=1476# road torque. This is a 64% improvement. And 1476 is just enough to initiate wheelspin.
If you had the 2.74 low gear and the stock TC, you would have 105x2.74x2.94=846#
It would take 1476/105=a 14.06 starter gear to equal what the TC can do. This would be 14.06/2.74=5.13 rear gears with the A998.

Ain't math grand. Even if the numbers are only approximate, you can clearly see the "approximate" results.
What I'm saying is; that to equal the take off power, of the 2800TC, and your current A904 with the 2.45 low in it, and your current 2.94s ..... would,
with the stock 1750TC, and the 2.74 low gear trans, still require; 5.13 rear gears!
But lets say you don't need that much take off performance. Ok lets look at a 2500TC. So you go to the 2500 rpm mark, which is 5000 on the chart, and follow that up to the red torque curve, and you will find about 200ftlbs@2500 there. And multiplying that by your current starter gear, we get 7.20x200=1440#, still enough to break traction, at least on the size I'm imagining that you are running, namely 255s or less.And finally, to equal 1440 with a 2.74low, and the current TC 125ftlbs would take ;
1440/(125x2.74)=4.20 rear gear.
The 2.94s will cruise at 65=2520@zeroslip (25.5tall tires), so the 2500 is a good match
 
Last edited:
AJ/FormS said:
If you had the 2.74 low gear and the stock TC, you would have 105x2.74x2.94=846#
It would take 1476/105=a 14.06 starter gear to equal what the TC can do. This would be 14.06/2.74=5.13 rear gears with the A998.
Click to expand...
You lost me here
 
Dana67Dart said:
You lost me here
Click to expand...

OK, Imagine the graph is accurate, and that when you floor it off the line, with the current combo, you unleash all 125 ftlbs of torque, right there at zero mph. (BTW that's 42hp.) Now your transmission is gonna multiply that by the low gear ratio, of 2.45, and your rear end is gonna further multiply that by 2.94.That's called TM; short for Torque Multiplication. So that looks like ; 125x2.45x2.94 =900ftlbs available. if you take of gently, you might use 30 to 50% of that, but if you floor it, that's all you got at 1750 rpm. Ok now remember that 900@1750.
Now you are thinking of installing the 2.74 low gear set., so let's do the math on that. Your engine still has the same 125Ft lbs and the rear is the same so; 125x2.74x2.94=1007ftlbs.
Now it takes somewhere around 1400 to break two rear tires loose, skinny ones maybe less.
So, you are still 400 short. You spent a lotta money and did a lotta work, and you got 106 ftlbs for all your skinned knuckles, and you still can't spin the tires! Dad gummit!
So lets work the numbers backwards, this time.
Say you wanted to break the tires loose with less than full throttle. So lets target 1476 ftlbs. And we'll use the current trans with the 2.45 low and the current 2.94 rear gear. The question is ;how much torque will the 273 need to be making? Answer; 1476/(2.45x2.94)=204 ftlbs.(BTW that's now 93hp@2400rpm; more than double than at 1750)So now we go to the graph and find 204ftlbs, and I see it at 2400rpm;the point where 408 and 4800 cross.
So you install that 2400TC and you are all set to spin tires off the line. I can't say they will spin very far, cuz that chart shows you peak at 437/2=214ftlbs at about 5900/2=2950, and your engine has not got a lot of reserve.
But before you get all giggly, this graph is no-where near accurate for showing the torque of a 2bbl 273. I just used it for illustrative purposes.
But I have run a 2800 behind a 1973 low-compression 318, and I can tell you it easily broke 255/50-15s BFG T/As loose.....both of them,lol. But to be fair, I had 3.55s and the 2.74 low. So I needed just 1476/(3.55x2.74)=151 engine ftpounds. A 318 has no trouble making that at 2800,even at PartThrottle.
>And I can tell you that 340 A-bodies came with 3.23s and 2.45 lows, that's just 3.23x2.45=7.91TM, and they easily broke loose the skinny E70-14s that the factory put on them. So that would be 1476/7.91=186ftlbs required.
Your 273 can make that number. The 340 cars had 2250 stall TCs ,IIRC.
ok so, I hope that makes it a little easier to understand.

Happy HotRodding
 
Last edited:
Thanks for the clarification. crystal clear. Thanks

So what you are saying is now I just need a roots blower and some nitro-methane! (just kidding)

Have a good one
 
Dana67Dart said:
Thanks for the clarification. crystal clear. Thanks
So what you are saying is now I just need a roots blower and some nitro-methane! (just kidding)
Have a good one
Click to expand...

naw, for just a little more scoot off the line, a 2400 will do that. For some more serious tirespin, a 2600/2800will do it....2bbl and all......unless your exhaust is also restricted....
Whatever you decide
Happy HotRodding
 
@ AJ: Very interesting your calculations. What torque converter do you recommend for a stock 1972 LA 318 with 3.93 gears to spin the wheels? And where to get one from?
 
I just do the math.
As always; talk to the TC supplier. They have access to better information than I do.
___________________
I really liked my 2800, but I think a 2600might have been enough.I just wanted to spin 275s, and eventually 295s, and that was what the 2800 was for. I already had the 2.74 low, and 3.55s, so my starter gear was 3.55x2.74=9.73, which is equivalent to 3.97s with an A904 low gear of 2.45; and my hiway rpm was 2870@zero-slip, so I went for the bad-boy 2800. No regrets.
I actually bought that TC in about 1980 for a slanty powered 69 Barracuda. Boy-O-Boy that slanty jumped off the line pretty good. When I sold that car,I kept the TC. Then in 2000 I had a new application, my winter motor.
____________________________________
I can say tho, that with 3.93s already, your starter gear with an A904 is 3.93x2.45=9.65. so all you need to break loose say 255s is about 1400ftlbs, so
1400/9.65=155 ftlbs out of the 318. at 2400 that is71hp. at 2800, that is83hp
So I think either one will get the job done, using less than full throttle. Once the tires are broke loose, you can just run the rpm up to whatever you want, and let the tires catch up to the road.
So for showing off, the 2800 is great. but spinning ain't winning, lol. so you might have to ease it out at part throttle. That's typical with a street chassis.
The 2400 might let you use more throttle with a little less wheelspin.
The 2800 also lets the secondaries open on a vacuum secondary carb, and that kicks in some additional power earlier. And That is what I liked; massive wheelspin,lol. Ok not massive,exactly. What I really liked was the Thermoquad moan, accompanied by the BFG howling.

I assume you have an LSD? If not, you're gonna need one one first, else you'll just be peg-legging all the time, and that makes kicking the back out around turns....impossible; and that takes all the fun out of it.
 
Last edited:
Etor
The 1972-318 should still be the 9.5Scr rated engine, And that,in an A-body with 3.93s and not tubbed should already be spinning tires.
So, seeing as you are in Switzerland, and TCs are I imagine are non-refundable if you are not happy with it; I think the smartest thing for me to do is repeat what I earlier said; call the supplier.He will ask you a few questions about your combo, and get you the right TC the first time.
If you lived next door, I'd loan you my 2800.............................
Here's a trick to help you choose between the 2500 and the 3000.
Drive your car in first gear up to about 95% of your desired stall and gas it hard. Watch what the tach indicates. You want it to flash up to the desired stall rpm or a tiny bit more. 95% of 2500 is 2375. Of 3000 is 2850. If it doesn't flash to the proper desired rpm, adjust your MPH, until it does.
Then decide which blast-off you like better.

Here's the deal tho; the 318 has a pretty flat torque curve between 2500 and 3000. Imagine it makes 200ftlbs all the way. At 2500 this is 95hp. at 3000 this is 114. So that's 19hp between the marks.
If your 72 is the Hi-Compression model, imagine it makes 250ftlbs, then the numbers are 119 and 143hp; this is now a 24hp difference. These are maximums at WOT. You can of course get your 318 to deliver less than the maximum by not flooring it.lol.
At 65mph, your rpm might be about 3300, so either TC will be fine. It looks like a city car, so if I had only the two choices of 2500 and 3000 I would choose 3000,lol. but I really liked the 2800 with the 4bbl-low-C teener and about the same starter gear that you have.I'm at 3650 car and driver
 
AJ/FormS said:
Etor

.


Here's the deal tho; the 318 has a pretty flat torque curve between 2500 and 3000. Imagine it makes 200ftlbs all the way. At 2500 this is 95hp. at 3000 this is 114. So that's 19hp between the marks.
If your 72 is the Hi-Compression model, imagine it makes 250ftlbs, then the numbers are 119 and 143hp; this is now a 24hp difference. These are maximums at WOT. You can of course get your 318 to deliver less than the maximum by not flooring it.lol.
At 65mph, your rpm might be about 3300, so either TC will be fine. It looks like a city car, so if I had only the two choices of 2500 and 3000 I would choose 3000,lol. but I really liked the 2800 with the 4bbl-low-C teener and about the same starter gear that you have.I'm at 3650 car and driver
Click to expand...
You believe there is 20 hp difference between a stock LOW compression 318 and a stock HIGH compression 318?? Sorry, I ain't buying it. There isn't but about half a point of difference in compression on any of them.
 
You must log in or register to reply here.
-

Back
Top